BEAM POWER TETRODE
The designation “6V6” was given to a family of octal-based power amplifier vacuum tubes.
The tubes were available in a variety of form factors:
6V6: metal T-8 envelope
6V6-G: glass ST-14 ("Coke-bottle shape") envelope
6V6-GT: glass T-9 "Bantam" envelope shape
6V6-GTA: same as the 6V6-GT, improved warm-up time
The 6V6 became, after its introduction in 1936, the power output tube many manufacturers wanted in their better radio set designs, especially in automobile receivers. It's easy to see why – with its “beam-forming” electrode construction, it was capable of providing 5 watts audio power in single-ended (one tube) service, or nearly 15 watts in push-pull (two tubes) configuration, at an economical cost. Distortion figures were relatively low, also; as little as 3.5% in some circuits. For these reasons, the 6V6 soon supplanted the older output tubes, such as the 6F6, 6K6, and 6N7, in many designs. In the beam-power tube category, only the 6L6 was its superior.
There were other uses for this versatile tube as well. The 6V6-GT found its way into many television receivers as a vertical output amplifier, and also saw use as a voltage regulator in industrial service. In the 1950s, professional audio designers began to make use of the 6V6 for high-fidelity amplifiers; this ultimately resulted in a “premium” version of the tube, designated the 7408.
The success of the 6V6 family led to still other versions as vacuum tubes grew smaller. An improved version with a locking base was developed for later automobile receivers, type 7C5. Miniature versions were released, the 6AQ5 and 6AQ5-A; though these types, being smaller, were not capable of the larger 6V6's plate dissipation and power output.
Today, among antique radio fans and tube audiophiles, the 6V6 has lost none of its luster. Production figures throughout the years have ensured that there are plenty of 6V6-types available today. The 6V6GT is still being produced in Russia and mainland China (American production ended in the 1970s), and is still showing up in many modern audio amplifier designs.
RCA Receiving Tube Manual, 1940 edition
various audiophile websites